In general, waveguide type optical modulators in which an optical waveguide or a modulating electrode is formed onto a substrate having an electro-optic effect are widely used in optical communication fields or optical measurement fields. At the request of improvement of high-speed, high-capacity communication or optical measurement precision in this optical modulator, the development of an optical modulator stably operable even in a high-frequency band is required. Recently, optical modulators of more than several tens GHz have also been implemented.
When the optical modulator operates in the high-frequency band, a jitter value of an eye pattern is apt to be large in an optical output waveform from the optical modulator. There occurs the degradation of modulation properties including the degradation of waveform quality of an optical output signal or the reduction of an optical transmission distance.
In a result of keen research by the inventors, it has been found that noise included in a microwave signal operating the optical modulator is one of factors causing the jitter value to be large as described below.
An example of the optical modulator is shown in FIG. 1. An optical modulation element 1 of FIG. 1(a) is formed with an optical waveguide (not shown), a modulating electrode, and the like on a substrate having the electro-optic effect such as LiNbO3. The modulating electrode is constructed with a signal electrode 2, a ground electrode (not shown), and the like. The optical modulation element 1 is connected to an optical fiber 3 for receiving and emitting an optical wave.
A connection substrate 4 having an amplifier 8 and the like and a termination substrate 5 having a termination device 9 and the like are arranged around the optical modulation element 1. Along with the optical modulation element 1, the connection substrate 4 or the termination substrate 5 is accommodated within a case 10 and forms an optical modulator module.
For reference, an example of the optical modulator module using the connection substrate is disclosed in Patent Document 1.
[Patent Document 1] JP-A-2003-233043
A method of operating an optical modulator will be described. A microwave signal generated from a modulation signal source 6 is introduced into a GPO connector 7 corresponding to an input terminal of a case 10 and is transmitted form the associated connector to a signal input terminal 11 of a connection substrate 4 as shown in FIG. 1B.
In the connection substrate 4, the microwave signal is output to a signal output terminal 12 through an amplifier 8 or a functional element (not shown) for converting the microwave signal into various states.
Wire bonding is done between the signal output terminal 12 of the connection substrate and an electrode pad of the signal electrode 2 of an optical modulation element. The microwave signal output from the connection substrate 4 is continuously transmitted to the signal electrode 2. According to the microwave signal transmitted to the signal electrode 2, the optical wave propagating within the optical waveguide of the optical modulation element is optically modulated.
An additional electrode pad is provided on a terminal of the signal electrode 2. Similarly, wire bonding is done between the electrode pad and a signal introduction terminal 14 of the termination substrate as shown in FIG. 1C. Thus, the microwave signal is additionally transmitted from the signal electrode 2 to the termination substrate 5, and is absorbed by a termination device 9 provided within the termination substrate.
However, the inventors have found that a radiation mode 13 of a microwave is generated from a microwave signal input to the signal input terminal 11 in the connection substrate 4 as shown in FIG. 1B and the radiation mode 13 propagates through the connection substrate and is recombined with the microwave signal propagating through a signal line in the signal output terminal 12. The recombined radiation mode serves as noise in a modulation signal. This noise propagates through the signal electrode 2 of the optical modulation element, thereby degrading modulation properties of the optical modulator.
In the termination substrate 5 as shown in FIG. 1C, part of a microwave signal introduced into the termination device 9 is reflected by the termination device and generates a radiation mode 15 of the microwave. The radiation mode 15 is recombined with the signal introduction terminal 14 of the signal propagating through the termination substrate, and is propagated to the signal electrode 2, so that the microwave travels in a direction reverse to the conventional propagation direction. This radiation mode 15 also serves as noise in a modulation signal.
In the connection substrate 4, a radiation mode (not shown) of the microwave reflected by the signal output terminal 12 is generated. The microwave propagating through the signal electrode 2 in the reverse direction generates a radiation mode (not shown) in the signal output terminal 12. These radiation modes are recombined with the signal input terminal 11 of the signal propagating through the connection substrate and flow backward to the modulation signal source 6, thereby causing the operation of the optical signal source to be unstable.